Slow down audio in VSE
The only way to do this in Blender is to change the Audio Strip's pitch value. Its in the Strip's properties panel in the VSE (N key). But it won't change the length of the strip nor will it change the waveform for you to see where the sound has stretched to. I would suggest doing an Audio Mixdown after the pitch change and re-importing it.

But honestly the sound will change to much, why not use Audacity free Audio editing software, it is open and there are portable versions available. It will do speed changes much better than Blender.